Production of Biosolids by Autothermal Thermophilic Aerobic Digestion (ATAD) from a Municipal Sewage Sludge: the Polish Case Study

energies

Review Production of Biosolids by Autothermal Thermophilic Aerobic Digestion (ATAD) from a Municipal Sewage Sludge: The Polish Case Study

Izabela Bartkowska * , Paweł Biedka and Izabela Anna Tałałaj

Department of Water Supply and Sewerage, Bialystok University of Technology, Wiejska 45A Street, 15-351 Bialystok, Poland; [email protected] (P.B.); [email protected] (I.A.T.) * Correspondence: [email protected]; Tel.: +48-085-746-9649

Received: 30 September 2020; Accepted: 23 November 2020; Published: 27 November 2020

Abstract: This manuscript analyzed the process of autothermal thermophilic aerobic digestion (ATAD) used in installations of municipal sewage treatment plants in Poland. Additionally, solutions for sludge management and the parameters of operating installations were presented. Attention was also put to the energy consumption of the process, where the energy consumption for 1 m3 of sludge treated was between 18.4 and 27.79 kWh. The amount of sewage ﬂowing into the analyzed plants was between 1500 and 14,000 m3/d. On the basis of research carried out in the years 2003–2019 in the selected plants, the characteristics of sludge after the ATAD process were presented. The parameters that determine the usefulness of the sludge as an organic fertilizer were indicated above all. The content of total nitrogen, which was from 2.4 to 8.1% of dry matter, ammonium nitrogen, which was from 0.8 to 1.8% of dry matter, and total phosphorus, which was from 1.1 to 4.2% of dry matter, recommended using sludge for fertilization. It was also pointed out that sewage sludge should be regularly tested for the dynamics of changes in chemical composition and biological parameters. These are the factors that increase risk and limit the use of sludge for fertilization.

Keywords: autothermal thermophilic aerobic digestion; ATAD; organic fertilizers; sewage sludge; natural utilization

1. Introduction Municipal wastewaterand sludge treatment plants are an essential element of surface water protection systems. However, they cause a real nuisance to the surrounding environment in terms of water and air quality, acoustic climate, or soil protection. One of the problems is the generation of a significant amount of different types of waste. These include municipal sewage sludge. The increasing amount of them forces the search for methods of their final disposal. It is estimated that in 2020, about 13 million tonnes of sewage sludge will be generated in European Union countries [1]. Sewage sludge is subject to the Directive of the European Parliament and of the Council of 19 November 2008 2008/98/EC on waste (the so-called waste framework directive). According to this directive, sludge is a waste and must be subject to a specific hierarchy of processing. First of all, it is necessary to prevent the generation of waste, continue to prepare it for reuse, recycle it, use other utylization methods, and finally dispose of it. However, it is not possible to prevent the formation of sewage sludge, as it is a type of waste that cannot be avoided [2]. Therefore, further priorities in the waste hierarchy are important, i.e., preparation for reuse or final disposal. The recycling of sewage sludge is already regulated by the previous directive 86/278/EEC of 12 June 1986. According to this directive, sludge can be used on the ground in a way that protects plants, soil, and groundwater as well as animal and human health. Closed-loop recycling schemes and new

Energies 2020, 13, 6258; doi:10.3390/en13236258 www.mdpi.com/journal/energies Energies 2020, 13, 6258 2 of 14

European legislation encourage the agricultural use of sewage sludge. This is a method of low-cost sludge disposal [1]. For the sake of circular economy (CE) policy, one of the key elements of a resource-eﬃcient and environmentally friendly society is recycling, which reduces costs and dependence on natural resources, reduces waste generation, and stimulates economic growth. From this point of view, sludge can be valued as a secondary raw material and used as a valuable source of nutrients and organic matter [3]. However, the reuse of sewage sludge is only possible if it is stabilized and sanitary safe. Therefore, studies are necessary on end-of-waste criteria for sludge-derived products [4]. One of the processes for the recovery of stabilized and hygienized sludge is autothermal thermophilic stabilization (ATAD). The aim of this study was to systematize the knowledge concerning the process of autothermal thermophilic stabilization of sewage sludge, which is the result of experience, resulting from participation in the design, installation, technological start-up, and operation of existing facilities. The presented material was developed on the basis of many years of research carried out in eleven municipal sewage treatment plants, in which sewage sludge is processed in the process of autothermal thermophilic stabilization. The review and evaluation of the results of the research will serve to realize that in accordance with the assumptions of sustainable development, the sewage sludge produced in the analyzed sewage treatment plants should be treated not as a waste but as a source of recovery of nutrients.

2. The Process Characteristic The thermophilic stabilization of sewage sludge is used mainly in Western European and North American countries. In Germany, this system is known as ATS (die aerob-thermophile Schlammbehandlung), in the United States and Canada, it is known as ATAD (autothermal thermophilic aerobic digestion). All of the above-mentioned process solutions are based on the natural phenomenon of exothermal oxidation of organic carbon with forced air supply. The thermophilic stabilization technology, which is used in treatment plants in Poland, was developed by Hubert K. E. Fuchs. In the 1970s, the first installation was built in Germany in Vilsbiburg, which is still operating today. Outside Western Europe, such plants operate in the USA, Canada, and Japan [5]. An essential role in the process of thermophilic stabilization is played by microorganisms, which enable the decomposition of organic compounds contained in sewage sludge. It basically takes place in two stages. The hydrolysis of complex organic compounds (proteins, carbohydrates and fats) as well as cell hydrolysis of dying microorganisms occurs in the first stage. These phenomena are caused by extracellular enzymes produced by thermophilic bacteria. In the second stage, thermophilic microorganisms oxidize the water-soluble hydrolysis products to low-energy compounds. These reactions are accompanied by heat release, and the final substances of these changes are CO2, H2O, and NH3 [6–8]. The process is shown in Figure1. The composition of the thermophilic process microflora differs from conventional activated sludge. First of all, it does not contain nitrifying bacteria, flocculating organisms, protozoa, and other forms of life [9]. The thermophilic microbial population replaces the organisms of the raw sludge. It is a fairly homogeneous microflora, which more than 95% consists of Bacillus, Thermus, or Actiomycestes [9,10]. Most strains belong to the species Bacillus stearothermophilus, which are active in the temperature range 40–80 ◦C. The remaining microflora are usually mixed cultures. In addition to the typical thermophilic microflora, there are thermotolerant mesophilic and extremely thermophilic bacteria, including the genus Thermus [9,11]. Usually, they have the ability to reproduce quickly, and the demise of thermophilic cells is equally rapid with the simultaneously occurring autolysis process. Energies 2020, 13, x FOR PEER REVIEW 2 of 14

European legislation encourage the agricultural use of sewage sludge. This is a method of low-cost sludge disposal [1]. For the sake of circular economy (CE) policy, one of the key elements of a resource-efficient and environmentally friendly society is recycling, which reduces costs and dependence on natural resources, reduces waste generation, and stimulates economic growth. From this point of view, sludge can be valued as a secondary raw material and used as a valuable source of nutrients and organic matter [3]. However, the reuse of sewage sludge is only possible if it is stabilized and sanitary safe. Therefore, studies are necessary on end-of-waste criteria for sludge-derived products [4]. One of the processes for the recovery of stabilized and hygienized sludge is autothermal thermophilic stabilization (ATAD). The aim of this study was to systematize the knowledge concerning the process of autothermal thermophilic stabilization of sewage sludge, which is the result of experience, resulting from participation in the design, installation, technological start-up, and operation of existing facilities. The presented material was developed on the basis of many years of research carried out in eleven municipal sewage treatment plants, in which sewage sludge is processed in the process of autothermal thermophilic stabilization. The review and evaluation of the results of the research will serve to realize that in accordance with the assumptions of sustainable development, the sewage sludge produced in the analyzed sewage treatment plants should be treated not as a waste but as a source of recovery of nutrients.

FigureFigure 1. The autothermal process process of of thermophilic thermophilic digestion. digestion.

The primary source of carbon and energy for thermophilic bacteria are fatty acids from hydrolysis products, while amino acids are their main source of nitrogen. Thermophilic bacteria are also resistant to the action of products of metabolism and have the ability to adapt to small amounts of oxygen. The intensive increase in the number of thermophilic bacteria, without the need to inoculate the sludge with a specific microflora, the rapid biodegradation of the substrate, the low sensitivity to temperature fluctuations, and the resistance to periodic interruptions in the aeration of the system guarantee even in emergency situations an extremely stable and flexible biological system in a thermophilic stabilization environment [12]. The processes taking place during autothermal thermophilic stabilization are in a way a continuation of the processes taking place in the active sludge, in which the oxidative biodegradation of organic substances and their mineralization takes place and parallel to the biosynthesis of cell substances. The role of aerobic heterotrophic microorganisms is taken over by thermophilic microflora. Direct oxidation of the biodegradable matter to carbon dioxide and water continues. The decomposed substances are used to synthesize new organisms, and then, the cellular material is oxidized. The heat generation in the ATAD reactor depends on the amount of organic matter contained in the sewage sludge. As the biological load increases, the temperature increases as well. However, increasing the load with the biodegradable organic matter does not cause a proportional increase in sludge temperature. First of all, the process becomes a system with limited oxygen transfer. The rate of oxygen uptake by thermophilic organisms is very high. This results in a reduction in the amount of oxidized organic substances and a consequent temperature stabilization. In addition, temperature has a significant influence on the amount and activity of thermophilic microflora. It was found that temperature increase above 65 ◦C, caused by increasing load on the system, inhibit the life processes of thermophilic microflora [6,8,13,14]. This reduces the amount of organic compounds removed and at the same time stabilizes the process temperature. These relationships can be described as self-regulation, which makes the system stable. Thermophilic microorganisms need more energy than e.g., mesophilic ones to maintain their life processes. Of the total amount of energy that is generated during the ATAD process, most of it is used during the metabolic changes of the microorganisms. Much less energy is used to build new cells. For this reason, processes occurring at higher temperatures are characterized by relatively low biomass production. With the development of the thermophilic bacteria population, the foaming of the sewage sludge appears. The dying cells of these microorganisms release intracellular matter, causing a reduction in surface tension, contributing to the formation of significant amounts of foam. The formation of foam indicates that the process proceeds properly. The foam layer improves oxygen transmission, which effectively increases process efficiency and biological activity. Foam is produced in the form of cell proteins, and oily and fatty substances are destroyed. It is within the foam that the highest biological activity is observed [15,16]. Significant biological activity within the foam layer contributes Energies 2020, 13, 6258 4 of 14 to the high oxygen utilization efficiency. In addition, it provides an insulating layer above the surface of the liquid that helps maintain the temperature of the sludge. However, for operational reasons, the foam formation process must be controlled, in particular its thickness and, in a sense, its density. The foam layer of a certain height and density is kept continuously due to the use of mechanical devices. Energies 2020, 13, x FOR PEER REVIEW 4 of 14 The air discharged from the autothermic thermophilic stabilization chambers contains compounds that areThe odor-intensive air discharged and bothersome.from the autothermic It is mainly thermophilic ammonia and stabilization products ofchambers decomposition contains and includescompounds the incomplete that are odor-intensive oxidation of organic and bothersome. substances It [17 is]. Ammoniamainly ammonia is created and as products a result ofof the ammonificationdecomposition ofand proteins includes and the is incomplete the main causeoxidation of the of organic nuisance substances smell of the[17]. process. Ammonia In is addition created to ammonia,as a result reduced of the ammonification sulfur compounds of proteins (e.g., hydrogen and is the sulfide, main cause mercaptans, of the nuisance dimethyl smell sulfide), of the aldehydes,process. ketones,In addition volatile to ammonia, fatty acids, reduced thialcohols, sulfur and compound others [s18 (e.g.,,19]) hydrogen are observed. sulfide, The mercaptans, formation ofdimethyl offensive odorsulfide), compounds aldehydes, is favored ketones, by volatile the oxygen fatty deficitacids, thialcohols, caused by the and high others temperature [18,19]) are of observed. the process The and theformation associated of low offensive redox potentialodor compounds [20]. is favored by the oxygen deficit caused by the high temperature of the process and the associated low redox potential [20]. 2.1. Technical Process Solution 2.1. Technical Process Solution The process of autothermal thermophilic stabilization is usually carried out in tanks connected in series.The The process installation of autothermal of two or morethermophilic cylindrical stabilization tanks made is usually of steel, carried concrete, out in ortanks plastic connected are used. Inin order series. to minimizeThe installation heat loss, of two the or reactors more cylindric are thermallyal tanks insulated made of steel, and closed.concrete, The or designplastic are of aused. typical ATADIn order plant to reactorminimize is shownheat loss, in the Figure reactors2. are thermally insulated and closed. The design of a typical ATAD plant reactor is shown in Figure 2.

FigureFigure 2. 2.Operation Operation ofof thethe autothermalautothermal thermophilic aerobic aerobic digestion digestion (ATAD) (ATAD) reactor reactor [21]. [21 ].

WithWith the the tanks tanks connected connected in in series, series, thethe temperature in in the the first ﬁrst stage stage of of the the installation installation is obtained is obtained inin the the lower lower thermophilic thermophilic decomposition decomposition range range (40–48 (40–48◦C), °C), with with maximum maximum disinfection disinfection and theand highest the temperaturehighest temperature is obtained is obtained in the last in stagethe last (55–60 stage◦ (55–60C) [22 ].°C) The [22]. daily The dischargedaily discharge of inactivated of inactivated sludge takessludge place takes only place from only the from last stage. the last With stage. the With end the of the end next of the discharge next discharge completed, completed, raw sludge raw sludge is fed to theis ﬁrst fed stage,to the first while stage, the partiallywhile the treated partially sludge treated is movedsludge is to moved the next to reactor. the next After reactor. feeding After thefeeding sludge, thethe reactors sludge, remain the reactors isolated remain for 23isolated h, at which for 23 pointh, at which thermophilic point thermophilic decomposition decomposition occurs. In occurs. order to limitIn order the temperature to limit the temperature rise, a heat exchanger rise, a heat can exchan be installedger can be in installed the last in reactor. the last A reactor. diagram A ofdiagram a sludge of a sludge node with ATAD installation is shown in Figure 3. node with ATAD installation is shown in Figure3.

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FigureFigure 3. 3. DiagramDiagram of of a a sludge sludge node node with with autothermal autothermal thermophilic thermophilic aerobic aerobic digestion (ATAD) [[21].21].

The ATAD installationinstallation maymay also also consist consist of of three three reactors. reactors. Three Three reactors reactors enable enable the the installation installation to tooperate operate in in two- two- and and three-stage three-stage systems. systems. In In atwo-stage a two-stage system, system, two two of of the the reactors reactors work work in in parallel parallel as asthe the first first stage, stage, and and the the third third as theas the second second stage. stage. In In a three-stage a three-stage system, system, reactors reactors operate operate in seriesin series as assuccessive successive stages. stages. Side and central aerators ensure a constant supply of air and effective effective mixing of the reaction chamber contents. In In some some solutions, solutions, oxygen oxygen is is intr introducedoduced into into the the reactors, reactors, alone or simultaneously simultaneously with air, thusthus slightlyslightly improvingimproving thethe process,process, butbut significantlysignificantly increasingincreasing itsits operatingoperating costs.costs. The important thing is to regulate the amount of foam produced. This process does not aim to completely eliminateeliminate the the foam, foam, as as its presenceits presence brings brings a number a number of benefits of benefits by using by e.g., using foam e.g., breakers, foam breakers,thus controlling thus controlling the thickness the and thickness density and of the density foam layer.of the Thefoam mechanical layer. The operation mechanical of these operation rotating of theseunits cutsrotating the foam,units cuts which the is foam, then dischargedwhich is then to thedisc surfaceharged of to the the sludge surface in of the the reactor sludge as in a the liquid reactor and asmixed a liquid with and the entiremixed content. with the The entire formation content. of foamThe formation should not of be foam regulated should by not limiting be regulated the amount by limitingof air supplied, the amount as there ofis air a risksupplied, of oxygen as deficiencythere is a andrisk increasedof oxygen odor deficiency nuisance and of sludge increased and gasesodor nuisancecaused by of the sludge production and gases of excessive caused amounts by the ofpr fermentationoduction of products.excessive amounts of fermentation products.The thermophilic stabilization system requires a system for cleaning the polluted air discharged fromThe the thermophilic reactors before stabilization it is released system into requires the atmosphere. a system for The cleani mostng commonlythe polluted used air discharged are water fromscrubbers the reactors (to remove before mainly it is ammonia), released biofilters,into the atmosphere. and chambers The where most the commonly photocatalytic used oxidation are water of scrubbersodor generating (to remove substances mainly takes ammonia), place. biofilters, and chambers where the photocatalytic oxidation of odor generating substances takes place. 2.2. Implemented Sludge Management Solutions 2.2. ImplementedThe process Sludge of autothermal Management thermophilic Solutions sludge stabilization is of fundamental importance in the management of a sewage treatment plant. Before being fed into the reactors, the sludge must be The process of autothermal thermophilic sludge stabilization is of fundamental importance in thickened and collected in sufficient quantity. In turn, processed sludge needs to be cooled down the management of a sewage treatment plant. Before being fed into the reactors, the sludge must be before it can be finally dewatered. In addition, the air from space above the sludge must be purified thickened and collected in sufficient quantity. In turn, processed sludge needs to be cooled down before it can is discharged be finally dewatered. into the atmosphere. In addition, All th ofe air this from requires space functionally above the sludge designed must and be purified efficient beforefacilities it andis discharged equipment. into the atmosphere. All of this requires functionally designed and efficient facilitiesObtaining and equipment. the required degree of sludge density is not possible as a result of gravity forces alone. MechanicalObtaining devices the required are used degree in all theof sludge plants density in question. is not Mostpossible often, as a the result sludge of gravity thickening forces process alone. Mechanicaltakes place indevices centrifuges are used (in case in all of the nine plants sewage in treatmentquestion. plants).Most often, In other the sl facilities,udge thickening a table and process drum takesthickener place was in used.centrifuges The condensed (in case of sewage nine sewage sludge goestreatment to the plants). ATAD installation,In other facilities, which isa discussedtable and drumin detail thickener below. was used. The condensed sewage sludge goes to the ATAD installation, which is discussed in detail below.

Energies 2020, 13, 6258 6 of 14

Energies 2020, 13, x FOR PEER REVIEW 6 of 14 The temperature of sludge discharged from the ATAD installation is high: up to 55–60 ◦C. It must be cooledThe downtemperature before beingof sludge fed intodischarged the drainage from unit.the AT RecoveredAD installation heat energy is high: can up be usedto 55–60 to heat °C. upIt themust compacted be cooled sludge down orbefore in other being sludge fed into treatment the drainage facilities. unit. Recovered heat energy can be used to heatIn up the the discussed compacted sludge sludge treatment or in other plants sludge and treatment centrifuges facilities. (in nine treatment plants), a ﬁlter press and aIn dewatering the discussed press sludge were treatment used for dewatering.plants and centri Dehydratedfuges (in sludgenine treatment is transported plants), to a filter the storage press yard.and a Sludge dewatering thickening press andwere dewatering used for dewatering. equipment Dehydrated are located sludge in the sludgeis transported mechanical to the treatment storage building.yard. Sludge In order thickening to optimize and thedewatering operation equipment of sludge managementare located in equipmentthe sludge inmechanical the analyzed treatment sewage treatmentbuilding. plants,In order multifunctional to optimize the tanks operation were made.of sludge In suchmanagement tanks, sludge equipment is collected in the beforeanalyzed the thickeningsewage treatment process. plants, The ATAD multifunctional sludge is also tanks discharged were made. into In the such multi-purpose tanks, sludge tank is collected for cooling. before the thickeningThe treatment process. plants The in ATAD question sludge use water is also scrubbers discharged to removeinto the mainlymulti-purpose ammonia tank and for devices cooling. in whichThe the treatment photocatalytic plants oxidation in question (PCO) use ofwater substances scrubbers responsible to remove for mainly odor ammonia formation and takes devices place. Thein which process the is photocatalytic shown in Figure oxidation4. (PCO) of substances responsible for odor formation takes place. The process is shown in Figure 4.

Figure 4. Figure 4. SchemeScheme of photocatalytic oxidation [23]. [23]. The process is particularly eﬀective for oxidizing pollutants and changing stench loads. Limited The process is particularly effective for oxidizing pollutants and changing stench loads. Limited requirements for the operation and control of photoionization, small overall size of the installation, requirements for the operation and control of photoionization, small overall size of the installation, combined with its high eﬃciency, decided to use this process in the sewage treatment plants in question. combined with its high efficiency, decided to use this process in the sewage treatment plants in 2.3.question. Selected Technical and Technological Parameters of the Implemented Installations

2.3. SinceSelected 2003, Technical 11 installations and Technological of autothermal Parameters thermophilic of the Implemented sediment Installations stabilization have been built in Poland. All of them operate in municipal sewage treatment plants. They have been designed for Since 2003, 11 installations of autothermal thermophilic sediment stabilization have been built various sizes of treatment plants with a capacity of 1500 to 14,000 m3/d. The range of wastewater in Poland. All of them operate in municipal sewage treatment plants. They have been designed for treatment plant capacity in terms of population equivalent ranges from 8900 to 77,900. The amount of various sizes of treatment plants with a capacity of 1500 to 14,000 m3/d. The range of wastewater sludge produced in these plants ranges from 800 to 5000 kg of dry matter per day. treatment plant capacity in terms of population equivalent ranges from 8900 to 77,900. The amount Sludge generated during wastewater treatment is highly hydrated and contains a small amount of sludge produced in these plants ranges from 800 to 5000 kg of dry matter per day. of totalSludge solids. generated For the autothermic during wastewater stabilization treatment process, is highly a pre-compaction hydrated and ofcontains the sludge a small toabout amount 5% ofof total total solids solids. is For required, the autothermic resulting stabilization in higher unit proc contentsess, a pre-compaction of volatile solids, of the which sludge should to about be 65 5% to 70%of total of total solids solids is required, and not lessresulting than 40in ghigher/L, which unit iscontents expressed of volatile as a chemical solids, oxygenwhich should demand be (COD). 65 to By70% keeping of total the solids dry massand not contents less than in the 40 sludgeg/L, which fed to is theexpressed ﬁrst stage as rectora chemical at this oxygen level, manydemand operational (COD). problemsBy keeping are the avoided. dry mass The contents reactors arein the fed sludge in batches fed onceto the per first day. stage In these rector wastewater at this level, treatment many 3 plants,operational the amount problems of sludgeare avoided. feed rangesThe reactors from 17are to fed 97 in m batches/d. Selected once per technical day. In andthese technological wastewater parameterstreatment plants, of the reactors the amount are shown of sludge in Tables feed1 andranges2. from 17 to 97 m3/d. Selected technical and technological parameters of the reactors are shown in Tables 1 and 2.

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Table 1. Technical and technological parameters of two-stage reactors.

Average Reactor Dimensions (m) Subsequent Reactor Daily Plant Hydraulic Treatment Population Filling Useful Capacity Retention Plants Equivalent Internal Total Height Capacity ATAD Diameter Height with 3 Time (d) (WWTP) 3 (m ) (m /d) Sludge WWTP_a 77,200 135 8.40 4.00 3.35 185 8.2 WWTP_b 22,690 34 7.71 3.60 2.80 130 7.65 WWTP_c 28,300 40 8.50 3.50 2.70 153 7.66 WWTP_d 35,000 27 8.00 3.30 2.50 125 9.3 9.20 3.52 2.70 180 WWTP_e 10,500 30 9.3 6.50 1 3.52 1 2.70 1 90 WWTP_f 39,900 46 9.42 3.80 3.00 209 9.1 WWTP_g 11,300 16.7 6.00 3.40 2.60 73.5 8.8 1 Second-stage reactor dimensions.

In the plants listed in Table1, the reactors operate in series as a two-stage system. The WWTP_a plant consists of six reactors operating in three process lines of two reactors each connected also in series. In the WWTP_e plant, the ﬁrst stage reactor has doubled capacity. This allows for an increased intensity of organic matter decomposition in this reactor to obtain higher temperatures.

Table 2. Technical and technological parameters of three-reactor installations.

Subsequent Average Reactor Sizes (m) Useful Sludge Daily Plant Hydraulic Population Sludge Reactor Treatment Capacity Internal Total Retention Plants Equivalent ATAD Filling Capacity Diameter Height 3 Time (d) (WWTP) (m3/d) Height (m ) WWTP_h 22,600 58 8.57 3.80 3.00 173 8.95 WWTP_i 36,200 96.6 9.43 4.36 3.50 244 7.6 WWTP_j 28,800 45.5 8.35 3.50 2.50 137 9.0 WWTP_k 35,000 70 8.50 3.80 3.00 170 7.3

The plants listed in Table2 are ATAD plants consisting of three reactors. The technology of the autothermal thermophilic stabilization process requires a continuous supply of air (oxygen), as the microorganisms adsorb oxygen supplied to the reactors very quickly. Therefore, it is important to equip the plant with eﬃcient and eﬀective aeration equipment. The basic technological equipment of the reactors are as follows:

Central aerators for the mixing and aeration of sludge in the reactors; these aerators are fixed • to the operating holes in the reactor floor slab; there is no more than one central aerator in each reactor; the speed is 950 or 1460 rpm; the rated motor power of the central aerator is 4.0 or 5.5 kW; Helical aerators, which are primarily designed to aerate the sludge and partially mix it; these • devices are mounted on the walls of the reactors; each reactor has two to four side aerators; the speed is 1445, 1460, or 1465 rpm; the rated motor power of the helical aerator motor is 4.0, 5.5, or 7.5 kW; Foam breakers (controllers) designed to limit the formation of coats and excess foam on the • boundary between the sludge and air; the breakers, similar to the central aerators, are mounted to the operating holes in the reactor floor slab; the number of foam controllers is two to eight; the speed is 920 rpm; the rated motor power is 1.1 kW; Internal tube heat exchangers, installed only in the final stage of the installation or in each tank, • for possible use of excess energy for the plant’s own needs; Energies 2020, 13, 6258 8 of 14

Control and measuring equipment, consisting of two temperature sensors (one at the 1.0 m level, • the other at the 2.0 m level above the tank bottom) and a level sensor (mounted at 0.3 m above the tank bottom); Shut-oﬀ ﬁttings—gate valves and throttles with actuators. • The total installed power for ATAD in the analyzed sewage treatment plants is presented in Table3.

Table 3. Total installed power for ATAD in the analyzed sewage treatment plants.

Subsequent Sludge Treatment Plants (WWTP) Total Installed Power (kW) WWTP_a 163.2 WWTP_b 49.8 WWTP_c 54.2 WWTP_d 44.0 WWTP_e 45.0 WWTP_f 64.3 WWTP_g 20.4 WWTP_h 94.1 WWTP_i 125.4 WWTP_j 62.7 WWTP_k 103.8

Properly functioning sludge management using the process of autothermal thermophilic stabilization requires the use of equipment for the pre-compaction of sludge. Moreover, according to Polish regulations, it is necessary to use equipment for the dewatering of sludge and elimination of odor nuisance.

3. Process Energy Consumption Due to the high rate of oxygen absorption by thermophilic organisms, there are high energy requirements for reactor aeration. An annual energy consumption balance for one of the sewage treatment plants is shown in Table4.

Table 4. Annual electricity consumption balance sheet.

Electricity Consumption (kWh) Total Electricity Measurement In the Sludge For Sludge By the ATAD Consumption Period Drainage Puriﬁcation Installation (kWh) Building January 99,732 94,980 22,560 217,272 February 97,530 83,340 19,560 200,430 March 90,809 88,020 23,160 201,989 April 138,259 58,200 18,240 214,699 May 91,962 87,300 26,640 205,902 June 88,607 71,400 21,120 181,127 July 135,844 42,840 18,840 197,524 August 94,369 55,620 18,120 168,109 September 86,220 66,240 19,200 171,660 October 94,219 52,920 19,920 167,059 November 110,381 42,300 16,440 169,121 December 112,988 62,100 21,000 196,088 Total: 1,240,920 805,260 244,800 2,290,980

The percentage share of individual technological processes in the total annual electricity consumption in one of the sewage treatment plants is shown in Figure5. Energies 2020, 13, x FOR PEER REVIEW 9 of 14

EnergiesThe2020 percentage, 13, 6258 share of individual technological processes in the total annual electricity9 of 14 consumption in one of the sewage treatment plants is shown in Figure 5.

11%

54% 35%

Sludge Dewatering ATAD System Wastewater Treatment

Figure 5. Participation of technological processes in total annual electricity consumption [24]. Figure 5. Participation of technological processes in total annual electricity consumption [24]. The knowledge of the energy balance of a sewage treatment plant allows assessing the power consumptionThe knowledge by individual of the processesenergy balance by means of a of sewage technical treatment indicators plant giving allows the assessing amount of the electricity power consumption inby relation individual to 1 mprocesses3 of treated by sewage means sludge. of technical indicators giving the amount of 3 electricityIn the consumption examined ATAD in relation installations, to 1 m theof treated following sewage values sludge. of electricity consumption indicators wereIn determined: the examined ATAD installations, the following values of electricity consumption indicators were determined: - For 1 m3 of sludge to be treated 18.4 to 27.79 kWh; - For 1 m3 of sludge to be treated 18.4 to 27.79 kWh; - With regard to 1 kg of sludge dry matter 0.18 to 0.28 kWh. - With regard to 1 kg of sludge dry matter 0.18 to 0.28 kWh. Unfortunately, therethere is is little little information information in the in scientificthe scientific literature literature concerning concerning the energy the intensity energy ofintensity the ATAD of the process—of ATAD process—of course, apartcourse, from apart the from casual the remarkscasual remarks that it that is significant. it is significant. Rojas Rojas and othersand others [25] report[25] report that that the energythe energy consumption consumption rates rates of ATAD of ATAD installations installations operating operating in Spain in Spain and 3 Irelandand Ireland are 9 are to 15 9 kWhto 15/m kWh/mof sludge3 of sludge processed processed or 0.3 to or 0.5 0.3 kWh to/ kg0.5 ofkWh/kg sludge dryof sludge matter. dry According matter. toAccording Archer etto al. Archer [26] the et al. energy [26] the consumption energy consum rateption of ATAD rate installationsof ATAD inst wasallations 3.9 kWh was/kg 3.9 of kWh/kg sludge dryof sludge matter. dry matter. Other technologiestechnologies of of sludge sludge stabilization stabilization also also cause cause the increasethe increase of energy of energy consumption consumption in sewage in treatmentsewage treatment plants. Basedplants. on Based the energy on the consumptionenergy consum inption the treatment in the treatment plant for plant the processfor the process of oxygen of 3 stabilizationoxygen stabilization of sludge, of sludge, the analyzed the analyzed indicators indicators fluctuate fluctuate from 6.44 from to 13.256.44 to kWh 13.25/m kWh/m3and from and 2.96 from to 6.022.96 kWhto 6.02/kg kWh/kg of dry massof dry [27 mass]. [27]. The obvious disadvantage of the the ATAD ATAD process is the significant significant electricity electricity consumption consumption [28,29]. [28,29]. This isis due due to theto the high high requirements requirements for reactor for reacto aeration.r aeration. The rate ofThe oxygen rate absorptionof oxygen byabsorption thermophilic by organismsthermophilic is severalorganisms times is higherseveral than times in higher the case than of mesophilic in the caseorganisms. of mesophilic However, organisms. with However, the use of appropriatewith the use equipment of appropriate design equi andpment proper design operation and of proper the system, operation costs of can the be significantlysystem, costs reduced. can be Processessignificantly in thereduced. reactors Processes are accompanied in the reactors by the are release accompanied of heat, whichby the canrelease be partially of heat, which recovered can bybe thepartially use of recovered exchangers. by the use of exchangers. The ATAD processprocess is characterized by a very fast increase of thermophilic bacteria bacteria quantity, quantity, without the need to to inoculate inoculate the the sediments sediments with with th thee proper proper microflora. microflora. An An important important advantage advantage is isalso also low low sensitivity sensitivity to to temperature temperature fluctuations fluctuations an andd resistance resistance to to periodic periodic interruptions interruptions in system aeration. This guarantees an extremely stable and flexibleflexible biological system. 4. Characteristics of Sewage Sludge In the ATAD process, the sludge is transformed into biomass, which can be used in nature. According to Polish legislation, sludge used in agriculture or for land reclamation must meet certain

Energies 2020, 13, 6258 10 of 14 conditions. It cannot contain heavy metals in quantities exceeding the applicable limit values. The representative sample of sewage sludge shall not contain Salmonella bacteria or eggs of intestinal parasites Askaris sp., Trichuris sp., Toxocara sp. Qualitative studies of the sludge were carried out in the existing facilities in the years 2003–2019. The sludge after the ATAD process and dewatering is black–brown and earthy. The sludge hydration ranges from 70% to 83% depending on the equipment used. The pH value of the sludge ranges from 6.5 to 9.3. Detailed changes in the pH value during the ATAD process are presented by Bartkowska [30]. The organic fraction content in the sludge dry matter is determined by the eﬀectiveness of the stabilization process. Stabilized sludge studied in the analyzed period contained from 29% to 60% organic substances [31]. Detailed information on changes in the content of organic substances in sewage sludge subjected to the ATAD process is presented by Bartkowska [32]. The group of indicators characterizing the content of carbon-type organic pollutants includes chemical oxygen demand. The values of chemical oxygen demand in the analyzed period ranged from 51 to 262 g O2/L. Sewage sludge contains biogenic compounds. The quantity of nitrogen and phosphorus compounds in the sludge is important. These substances were determined on the basis of total nitrogen and ammonium nitrogen and total phosphorus concentration. The values of total nitrogen were from 2.4 to 8.1% of dry matter, while ammonium nitrogen was from 0.8 to 1.8% of dry matter, and total phosphorus was from 1.1 to 4.2% of dry matter. Other compounds found in sewage sludge are calcium and magnesium compounds. In the examined sludge, calcium compounds were present in the amount from 1.6 to 9.2% of dry matter. The amount of magnesium compounds was found at the level of 0.08 to 1.31% of dry matter. Sewage sludge may contain various amounts of heavy metals whose presence depends primarily on the share of industrial wastewater in the total amount of wastewater treated. During the research, small amounts of heavy metals were found. The observed ranges of values of the investigated heavy metals are presented in Table5.

Table 5. Concentration of heavy metal in the tested sludge.

Metal Range of Values (mg/kg of Dry Matter) Cadmium 0.25–6.12 Copper 68–806 Nickel 8.6–55.3 Lead 4.7–170.2 Zinc 328–2396 Mercury 0.19–9.30 Chromium 12–450

Within the analyzed period, the concentration of analyzed heavy metals did not exceed the values allowed by the Polish legislation. No significant fluctuation of their concentration in sludge was also found. The concentration of analyzed heavy metals in the examined sludge can range as follows: Zn > Cu > Cr > Pb > Ni > Cd > Hg. During the process of autothermal thermophilic sludge stabilization, the specific electrolytic conductivity increases. Its value in stabilized sludge ranged from 11,420 to 19,120 µS/cm. The results of the study of conductivity changes during the two-stage ATAD process are described by Bartkowska [33]. Higher redox potential was also found in stabilized sludge. Its value oscillated from 561.60 to − 306.40 mV. Changes in redox potential were presented by Bartkowska [20]. − When examining sewage sludge, particular attention was paid to the effectiveness of its stabilization. During the ATAD process, the content of easily decomposable organic substances is reduced. The minimization of occurring biochemical reactions results in limiting the decomposition of the remaining organic substances in an uncontrolled and burdensome way for the environment. This prevents the sludge from its natural putrescibility tendency and release odorogenic substances. Energies 2020, 13, x FOR PEER REVIEW 11 of 14 Energies 2020, 13, 6258 11 of 14

reduced. The minimization of occurring biochemical reactions results in limiting the decomposition Weof also the remaining achieve a visibleorganic reductionsubstances in in the an weightuncontrolled of the and sludge. burdensome There isway no for clear, the legallyenvironment. required definitionThis prevents of the the notion sludge of sedimentfrom its natural stabilization putresci inbility Poland. tendency The notionand release of the odorogenic so-called technicalsubstances. limit ofWe stabilization also achieve is useda visible to assess reduction the degreein the weight of sludge of the stabilization, sludge. There which is no causes clear, alegally reduction required in the initialdefinition dry organic of the notion matter of at sediment the level stabilization of 38–40% [in34 ,Poland.35]. During The notion the conducted of the so-called research, technical it was limit found of stabilization is used to assess the degree of sludge stabilization, which causes a reduction in the that the most numerous range of organic matter content decrease in sludge dry matter is from 37.84% initial dry organic matter at the level of 38–40% [34,35]. During the conducted research, it was found to 40.25%. Detailed results of the research concerning the decrease in dry matter content, organic that the most numerous range of organic matter content decrease in sludge dry matter is from 37.84% substances, and COD values are presented by Bartkowska [32]. to 40.25%. Detailed results of the research concerning the decrease in dry matter content, organic A very important task of the sludge treatment plant management is to reduce the risk of the substances, and COD values are presented by Bartkowska [32]. presenceA pathogenicvery important organisms. task of the Although sludge treatment they have plant limited management capacity to is survive to reduce in thethe environment,risk of the theirpresence resistance pathogenic varies. organisms. Survival is Although a specific they feature have characteristic limited capacity of each to survive microorganism in the environment, and is also a functiontheir resistance of many varies. factors. Survival One of is thema specific is the feature temperature, characteristic which of ineach a second microorganism stage reactor and is reaches also 55–60a function◦C. In of addition, many factors. in the One ATAD of plant,them is sludge the temperature, stays for a which minimum in a second of 23 h stage in each reactor of the reaches reactors, and55–60 the °C. hydraulic In addition, retention in the timeATAD of plant, sludge sludge in these stays plants for a isminimum on average of 23 7.3 h in to each 9.3 fullof the days. reactors, One of theand important the hydraulic issues retention connected time with of sludge the ATAD in these process plants is is the on provision average 7.3 of to a stabilized9.3 full days. sludge One of without the pathogensimportant [29 issues]. The connected repeated microbiologicalwith the ATAD testingprocess of is sludge the provision after the of ATAD a stabilized process sludge confirmed without that it waspathogens devoid of [29]. the The specific repeated DNA microbiological of Salmonella sp. testin andg live of eggssludge of after the parasites the ATADAscaris processsp., confirmedTrichuris sp., andthatToxocara it was sp.devoid According of the specific to research DNA by of L Salmonellaópez et al. [sp.36] and the aerobiclive eggs thermophilic of the parasites treatment Ascaris was sp., the bestTrichuris one also sp., reducing and Toxocara E. coli, sp. while According the influence to research of hydraulic by López retention et al. [36] times the aerobic to microbial thermophilic reduction wastreatment non-significant. was the best Thus, one thealso ATADreducing may E. coli, possess while high the potentialinfluence of for hydraulic reducing retention biological times risks to in sewagemicrobial sludge reduction [37]. was non-significant. Thus, the ATAD may possess high potential for reducing biological risks in sewage sludge [37]. 5. Quantities of Generated Sludge 5. Quantities of Generated Sludge In all analyzed wastewater treatment plants, sludge is processed in the ATAD plant. These are treatmentIn all plants analyzed of various wastewater sizes, trea wheretment the plants, average sludge annual is processed weight of in generated the ATAD sludge plant. These in 2018 are was betweentreatment 200 plants and 2479 of various Mg [38 sizes,]. The where weight the of averag the generatede annual weight sludge of is genera shownted in Figuresludge6 in. 2018 was between 200 and 2479 Mg [38]. The weight of the generated sludge is shown in Figure 6.

3000

2500

2000

1500 (Mg/year)

1000

500 average mass of sludge yearly produced sludge of average mass

0 WWTP_a WWTP_b WWTP_c WWTP_d WWTP_e WWTP_f WWTP_g WWTP_h WWTP_i WWTP_j WWTP_k mass of sludge 1550 1072 826 2172 421 625 200 1873 1752 1808 2479

FigureFigure 6. 6.Weight Weight of of dehydrated dehydrated sludge sludge generated generated in in 2018 2018 in in the the analyzed analyzed sewage sewage treatment treatment plants plants [38 ]. [38]. Each sewage treatment plant keeps a record and waste transfer cards, including the generated sewage sludge. In the analyzed treatment plants, the generated sewage sludge is used in 95–100% of the natural environment. Municipal sewage sludge is received and utilized by landowners in the surrounding area. The sludge is used on medium and heavy soils. Energies 2020, 13, 6258 12 of 14

6. Conclusions Studies carried out in functioning ATAD installations have confirmed the effectiveness of this process in the gaining of stabilized and hygienized sludge. The process allows obtaining sludge that does not putrify and is free of pathogenic microorganisms, parasites, and fungi. The sludge is sanitary and hygienically safe, while it contains fertilizers, which as a valuable commodity can successfully be returned to the natural cycle. The conducted analyses showed that sewage sludge can be a good substitute for mineral fertilizers, which, in addition to generating additional environmental problems, also require considerable investment costs. The fertilization of soils with sewage sludge is both economically beneficial and also necessary to restore and maintain ecological balance, as it closes the nutrient cycle in the entire local ecosystem. The best evidence of this is the growing interest of individual farmers in the use of sludge from the analyzed sewage treatment plants. In addition, a certain part of the wastewater treatment plants has been granted a decision by the minister of agriculture and rural development to market organic fertilizer produced from municipal sewage sludge in the ATAD process. Therefore, they should be treated not as a waste but as a source of nutrients recovery. The impact of the presented installations on the surrounding environment can be analyzed in terms of protection of groundwater and earth surface, protection of surface water, protection against waste, air protection, protection against noise and vibrations, or landscape protection. The use of tight reactor constructions and inter-objective cables ensures the protection of groundwater and the ground surface. Tanks, which are used, are hermetic, and the elimination of odors is achieved by using equipment for the photocatalytic oxidation of odor-generating agents. The noise emission generated by aeration equipment is not higher than 40–50 dB and does not exceed the boundaries of the plot of the plant. However, when choosing a sludge management solution based on the ATAD process, one has to take into account the higher operating costs resulting from the use of electricity to aerate the treated sludge.

Author Contributions: I.B., P.B. and I.A.T. conceived and designed the experiments; I.B., P.B. and I.A.T. analyzed the data; I.B. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: The research was carried out as part of research work plan WZ/WBiIS´/2/2019 at the Bialystok University of Technology and financed from a subsidy provided by the Minister of Science and Higher Education. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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